Radiant tubes are frequently used in natural gas burners or electrical heating elements, and are made of metal, ceramic or a combination thereof. Heat is passed through the tube to heat the load. Such radiant tubes have both industrial and household uses. In household uses, radiant tubes may be referred to as an heating elements and are found in numerous appliances.
A large variety of radiant tubes are used for industrial and commercial uses from heating facilities to incorporation in industrial processes. These radiant tubes may have a large variety of shapes and uses; however, they use the principals of radiant heat to transfer heat view infrared radiation. Radiant heat is transmitted by infrared radiation from a heat source, and not by conduction or convection.
Radiant tubes are used to better maintain a constant temperature. One type of conventional radiant tube heater has only a single tube. Another conventional radiant tube heater has in combination concentric inner and outer tubes. It is also known to bend or twist these embodiments to compact or concentrate the amount of heat provided by the radiant tube.
Two exemplary forms involve a straight radiant tube with either a reflector to reflect the heat as desired or at least one additional tube forming concentric tubes with spaces therebetween. For example, U.S. Pat. No. 6,971,871 discloses a radiant heater including a burner having an inlet for receiving an air and gas mixture and an exhaust for emitting exhaust gasses generated by combustion of the air and gas mixture within the burner with an elongated radiant heating tube having an inlet for receiving the exhaust by the burner. A reflector having a reverted U configuration in cross-section is suitably supported in spaced overlying relation and generally coextensive with the radiant tube. U.S. Pat. No. 4,979,491 shows a radiant tube and reflector hanger wherein the radiant tube is shown as a simple tube with opening therethrough and a shield disposed above the radiant tube directing the heat downward accordingly. U.S. Pat. No. 4,727,854 discloses a high efficiency infrared radiant energy heating system and a reflector therefor having an inner tube and an outer tube; the outer tube is closed at one end.
Similarly, U.S. Pat. No. 5,163,416 discloses a radiant tube arrangement for high temperature, industrial heat treat furnace in which the tube within a tube (or multiple configurations thereof) are used with the outer tube closed at the end opposite the source of heat fuel-fired ceramic heat tubes. U.S. Pat. No. 5,127,826 teaches a single ended radiant tube for furnace heating that has four concentric tubes. Fuel gas is fed through an inner tube with combustion air flowing through the space between tubes. The outer tube is closed at the end opposite the source of gases. U.S. Pat. No. 4,479,535 describes a recuperative radiant tube of the closed ended outer tube type.
Another example of the closed end outer tube is found in U.S. Pat. No. 5,016,610 which discloses a radiant tube type heater having inner and outer concentric tubes the inner tube delivering ignite fuel while the outer tube provides space for combustion gas (air) and has a closed end opposite the source of fuel. The tubes used therein have a structure in which silicon carbide particles are joined to each other by a sintering aid such as an aluminum compound sintering aid or a boron compound sintering aid.
Conversely, U.S. Pat. No. 5,224,542 discloses a gas fired radiant tube heater uses a gas fired burner which fires heated products of combustion into an inner longitudinally-extending tube that is concentrically pinned to an outer longitudinally-extending tube which is concentrically mounted within a heat transfer tube which radiates the heat to work. U.S. Pat. No. 4,800,866 teaches a low nitrous oxide radiant tube burner involving tubes within tubes. U.S. Pat. No. 6,321,743 shows a single-ended self-recuperated radiant tube annulus system. U.S. Pat. No. 5,224,857 shows a radiant tube arrangement for high temperature, industrial heat treat furnace in which the outer tube is closed off at one end thereof.
Examples of novel uses of radiant tubes include U.S. Pat. No. 6,489,553 which shows an at home cogeneration generator utilizing a gas heated radiant tube, U.S. Pat. No. 6,769,909 which shows a paint baking oven with radiant tube heating elements associated reflectors, U.S. Pat. No. 5,981,920 which shows a furnace for heating glass sheets uses electrical coils to heat the inside of parallel burner tubes, and U.S. Pat. No. 3,663,798 which shows a stove top having radiant tube heating elements disposed beneath a flat surface in which the tubes have a heating element and shield disposed within them so that the radiant tube heats primarily on the top side opposite the shield. Examples of household uses include U.S. Pat. No. 5,626,125 shows a space heating apparatus which includes a radiant tube bent back on itself forming parallel legs and shielding to direct the heat as desired disposed above each leg and a second type of shield disposed above both legs and shielding to direct the heat as desired.
Various shapes of radiant tubes are also known. The most common is probably the U-shaped configuration in which the radiant tubes foam legs and curves forming a U-shape. U.S. Pat. No. 4,673,350 shows such a radiant tube wherein the tube is bent back upon itself; however, no shield or outer tube is shown therein. U.S. Pat. No. 6,027,333 shows a U-shaped radiant tube burner.
Some radiant tubes, especially those which are heated by the combustion of air and gaseous hydrocarbon fuels may have a catalyst disposed within the radiant tube to regulate combustion. U.S. Pat. No. 5,992,409 teaches a catalytic radiant tube heater and method for its use having a catalyst tube disposed within the radiant tube. A tube having openings spaced throughout is disposed within the catalytic tube to deliver hydrocarbon fuels throughout the catalytic tube. The combustion of these hydrocarbons heats the catalytic tube which in turn heats the radiant tube. The preferred catalyst is cobalt chromium oxide spinel. A reflector is used in this design to direct the radiation in the desired direction.
Alternative cross sections of radiant tubes are also known. Such alternative cross sections may include a variety of configurations designed to produce eddies within the turbulent flow of air and/or gaseous fuel to facilitate a desired mixture thereof. Various openings and inlets may be provided at the open ends of the tubes to introduce different air and/or gaseous fuels from different openings and/or to produce a desirable mixture of combustion gases and fuels. It is also known to have inlets and openings for air and gaseous fuel to be introduced or exhausted. Furthermore, although the outer tube tends to be a single piece, the inner tube may be different tubes either concentrically included therein or lined up end to end within the outer tube. Fans and other devices to control or mix the flow of air and fuel are also used in conventional radiant tube applications and may be provided within the outer tube structure and at either the input or output of gaseous fuel and air.
It is conventional to use mounting devices to hold the radiant tube in place relative to the shield, or where appropriate, to additional concentric tubes, or both. Spacers may be used to hold the radiant tubes in place relative other concentric tubes to maintain proper relative spacing therebetween.